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Tailoring the Optical Response of III-V Nanowire Arrays
Lund University, Lund, Sweden.
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Semiconductor nanowires show a great deal of promise for applications in a wide range of important fields, including photovoltaics, biomedicine, and information technology. Developing these exciting applications is strongly dependent on understanding the fundamental properties of nanowires, such as their optical resonances and absorption spectra. In this thesis we explore optical absorption spectra of arrays of vertical III-V nanowires with a special emphasis on structures optimized to enhance absorption in the solar spectrum. First, we analyze experimentally determined absorption spectra of both indium phosphide (InP) and gallium phosphide (GaP) nanowire arrays. The study provides an intuitive understanding of how the observed absorption resonances in the nanowires may be tuned as a function of their geometrical parameters and crystal structure. As a consequence, the spectral position of absorption resonances can be precisely controlled through the nanowire diameter. However, the results highlight how the blue-shift in the optical absorption resonances as the diameter of the nanowires decreases comes to a halt at low diameters. The stop point is related to the behavior of the refractive indices of the nanowires. The wavelength of the stop is different for nanowire polytypes of similar dimensions due to differences in their refractive indices. We then present a theoretical argument that it is important to consider symmetry properties when tailoring the optical modes excited in the nanowires for enhanced absorption. We show that absorption spectra may be enhanced compared to vertical nanowires at normal incidence by tilting the nanowires with normal incidence light, or by using off-normal incidence with vertical nanowires. This is because additional optical modes inside the nanowires are excited when the symmetry is broken. Looking forward to omnidirectional applications, we consider branched nanowires as a way to enhance the absorption spectra at normal incidence by taking advantage of simultaneous excitation of the spectrally different optical modes in the branches and the stems. Third, we describe in theoretical terms how integrating distributed Bragg reflectors (DBRs) with the nanowires can improve absorption spectra compared to conventional nanowires. DBRs provide a way to employ light trapping mechanisms which increases the optical path length of the excited modes and thereby improves the absorption of the excited modes. At normal incidence, DBR-nanowires improve the absorption efficiency to 78%, compared to 72% for conventional nanowires. We show that the efficiency is increased to 85% for an off-normal incident angle of 50˚. Overall, our results show that studies of optical resonances in nanowires that take the light-matter interaction into account provide opportunities to develop novel optical and optoelectronic functionalities in nanoscience and nanotechnology.

Place, publisher, year, edition, pages
Lund: Lund University , 2017. , p. 63
Keywords [en]
III-V nanowires, absorption, optical modes, photovoltaics
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:hh:diva-36683Libris ID: 20806417ISBN: 978-91-7753-277-4 (print)ISBN: 978-91-7753-278-1 (electronic)OAI: oai:DiVA.org:hh-36683DiVA, id: diva2:1201486
Public defence
2017-06-02, Rydbergsalen, Fysicum, Sölvegatan 14, Lund, 13:15 (English)
Opponent
Supervisors
Available from: 2018-05-03 Created: 2018-04-25 Last updated: 2018-05-03Bibliographically approved
List of papers
1. Considering Symmetry Properties of InP Nanowire/Light Incidence Systems to Gain Broadband Absorption
Open this publication in new window or tab >>Considering Symmetry Properties of InP Nanowire/Light Incidence Systems to Gain Broadband Absorption
2017 (English)In: IEEE Photonics Journal, ISSN 1097-5764, E-ISSN 1943-0655, Vol. 9, no 3, article id 4501310Article in journal (Refereed) Published
Abstract [en]

Geometrically designed III-V nanowire arrays are promising candidates for disruptive optoelectronics due to the possibility of obtaining a strongly enhanced absorption resulting from nanophotonic resonance effects. With normally incident light on such vertical nanowire arrays, the absorption spectra exhibit peaks that originate from excitation of HE1m waveguide modes in the constituent nanowires. However, the absorption spectra typically show dips between the absorption peaks. Conventionally, such weak absorption has been counteracted by either making the nanowires longer or by decreasing the pitch of the array, both alternatives effectively increasing the volume of absorbing material in the array. Here, we first study two approaches for compensating the absorption dips by exciting additional Mie resonances: 1) oblique light incidence on vertical InP nanowire arrays and 2) normal light incidence on inclined InP nanowire arrays. We then show that branched nanowires offer a novel route to achieve broadband absorption by taking advantage of simultaneous excitations of Mie resonances in the branches and guided HE1m modes in the stem. Finite element method calculations show that the absorption efficiency is enhanced from 0.72 for vertical nanowires to 0.78 for branched nanowires under normal light incidence. Our work provides new insight for the development of novel efficient photovoltaics with high efficiency and reduced active material volume.

Place, publisher, year, edition, pages
Piscataway: IEEE, 2017
Keywords
Nanophotonics, nanowire arrays, absorption, guided modes, Mie resonances, photovoltaics
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:hh:diva-35599 (URN)10.1109/JPHOT.2017.2690313 (DOI)000400414300001 ()2-s2.0-85018356671 (Scopus ID)
Available from: 2017-12-05 Created: 2017-12-05 Last updated: 2018-04-25Bibliographically approved
2. Enhanced broadband absorption in nanowire arrays with integrated Bragg reflectors
Open this publication in new window or tab >>Enhanced broadband absorption in nanowire arrays with integrated Bragg reflectors
2018 (English)In: Nanophotonics, E-ISSN 2192-8614, Vol. 7, no 5, p. 819-825Article in journal (Refereed) Published
Abstract [en]

A near-unity unselective absorption spectrum is desirable for high-performance photovoltaics. Nanowire arrays are promising candidates for efficient solar cells due to nanophotonic absorption resonances in the solar spectrum. The absorption spectra, however, display undesired dips between the resonance peaks. To achieve improved unselective broadband absorption, we propose to enclose distributed Bragg reflectors (DBRs) in the bottom and top parts of indium phosphide (InP) nanowires, respectively. We theoretically show that by enclosing only two periods of In0.56Ga0.44As/InPDBRs, an unselective 78% absorption efficiency (72% for nanowires without DBRs)is obtained at normal incidence in the spectral range from 300 nm to 920 nm. Under oblique light incidence, the absorption efficiency is enhanced up to about 85% at an incidence angle of 50º. By increasing the number of DBR periods from two to five, the absorption efficiency is further enhanced up to 95% at normal incidence. In this work we calculated optical spectra for InP nanowires, but the results are expected to be valid for other direct band gap III-V semiconductor materials. We believe that our proposed idea of integrating DBRs in nanowires offers great potential for high-performance photovoltaic applications. ©2018 Håkan Pettersson et al., published by De Gruyter, Berlin/Boston.

Place, publisher, year, edition, pages
Berlin: De Gruyter Open, 2018
Keywords
light trapping, distributed Bragg reflectors (DBRs), nanowires, photovoltaics
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:hh:diva-35885 (URN)10.1515/nanoph-2017-0101 (DOI)2-s2.0-85045636459 (Scopus ID)
Funder
Swedish Research CouncilSwedish Foundation for Strategic Research Knut and Alice Wallenberg Foundation
Note

Funding: NanoLund, the Swedish Research Council (VR), the Swedish Foundation for Strategic Research (SSF), and the Knut and Alice Wallenberg Foundation

Available from: 2017-12-14 Created: 2017-12-14 Last updated: 2018-07-02Bibliographically approved
3. Bias-dependent spectral tuning in InP nanowire-based photodetectors
Open this publication in new window or tab >>Bias-dependent spectral tuning in InP nanowire-based photodetectors
Show others...
2017 (English)In: Nanotechnology, ISSN 0957-4484, E-ISSN 1361-6528, Vol. 28, no 11, article id 114006Article in journal (Refereed) Published
Abstract [en]

Nanowire array ensembles contacted in a vertical geometry are extensively studied and considered strong candidates for next generations of industrial scale optoelectronics. Key challenges in this development deal with optimization of the doping profile of the nanowires and the interface between nanowires and transparent top contact. Here we report on photodetection characteristics associated with doping profile variations in InP nanowire array photodetectors. Bias-dependent tuning of the spectral shape of the responsivity is observed which is attributed to a Schottky-like contact at the nanowire-ITO interface. Angular dependent responsivity measurements, compared with simulated absorption spectra, support this conclusion. Furthermore, electrical simulations unravel the role of possible self-gating effects in the nanowires induced by the ITO/SiOx wrap-gate geometry. Finally, we discuss possible reasons for the observed low saturation current at large forward biases.  

Place, publisher, year, edition, pages
Bristol: Institute of Physics Publishing (IOPP), 2017
Keywords
nanowires, nanowire arrays, IR photodetectors, solar cells, nanophotonics
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:hh:diva-32769 (URN)10.1088/1361-6528/aa5236 (DOI)000395937500001 ()2-s2.0-85014564717 (Scopus ID)
Available from: 2016-12-20 Created: 2016-12-20 Last updated: 2018-04-25Bibliographically approved

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  • apa
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